The panel announced Monday that it would appoint an investigative subcommittee to determine whether Mrs. Berkley violated the House ethics rules with her advocacy for a Las Vegas kidney center in which her husband had a financial stake.

The investigation threatens to derail the Berkley campaign just as the Democrat appeared to be making inroads in her contest against the Republican incumbent, Sen. Dean Heller. Polls have shown Mr. Heller’s lead shrinking in recent months, with a Public Policy Polling survey released June 12 putting him ahead by 44 percent to 43 percent, a razor-thin margin that is within the survey’s error margin.

“It is going to have some impact on the Senate race, hopefully not much,” said Craig Holman, government affairs lobbyist for the watchdog group Public Citizen. “It does provide Berkley’s opponents with fodder to use in the election, and they already have.”

He said the silver lining for Mrs. Berkley is that the committee is unlikely to announce its findings before the election, a standard practice in election years. Still, the cloud of a formal House ethics investigation is the last thing any candidate needs in a race this tight.

Going before the House Ethics Committee isn’t necessarily a career-ender. Last month, Rep. Charles B. Rangel won the New York Democratic primary after the panel censured him for committing 11 ethics violations. In 2010, Rep. Maxine Waters, California Democrat, coasted to victory after the committee launched an investigation of whether she steered federal stimulus funding to a bank with ties to her husband.

The difference is that Mr. Rangel and Mrs. Waters were running for re-election in House districts that overwhelmingly favor their party and in which they are entrenched incumbents. Mrs. Berkley enjoys some name recognition in Nevada as a seven-term congresswoman, but she’s challenging an incumbent senator in an evenly divided state.

Analysts say the Nevada race could swing the balance of power in the Senate. Democrats currently hold a four-seat majority but trail in a half-dozen races.

Mr. Heller, who was appointed in 2011 to fill the vacancy left by resigning Sen. John Ensign, had no comment on the decision, but plenty of other Republicans were happy to jump in with takes on Mrs. Berkley’s ethical woes.

“Since Berkley entered the political arena, we’ve seen a long pattern of ethical questions surrounding her conduct,” said Rob Jesmer, executive director of the National Republican Senatorial Committee, in a statement. “Nevadans deserve someone in the Senate who they can trust to work on their behalf and not someone like Ms. Berkley — who puts her own financial and political interests first.”

American Crossroads, the conservative independent-expenditure committee run by Republican campaign guru Karl Rove, had already launched television ads blasting Mrs. Berkley’s work on behalf of the kidney center at University Medical Center in Las Vegas.

Her husband, Dr. Larry Lehrner, runs a practice that has a contract with the center to provide a wide range of kidney care, including transplants. The conflict was first exposed in a September article in the New York Times.

“Shelley Berkley makes the system work for herself,” says the ad, which began running last month.

The Berkley campaign fired back with a television spot pointing out that Mr. Heller had also called for the center to remain open.

“Shelley Berkley worked with Dean Heller, standing up to Washington bureaucrats who wanted to close Nevada’s only kidney-transplant center,” says the Berkley ad. “This was about saving lives.”